This section is intended to provide a background to the various embodiments of the technology described in this disclosure. The description in this section may include concepts that could be pursued, but are not necessarily ones that have been previously conceived or pursued. Therefore, unless otherwise indicated herein, what is described in this section is not prior art to the description and/or claims of this disclosure and is not admitted to be prior art by the mere inclusion in this section.
In order to enhance the capacity of wireless communication systems, it has been agreed that future wireless communication systems, such as the fifth generation (5G) wireless communication systems, shall operate at higher frequencies as compared with conventional wireless communication systems, such as the third generation (3G) wireless communication systems.
However, due to reduced ability of radio signals at high frequencies to diffract around objects, the radio signal path between a terminal device (such as a user equipment (UE)) and its SAN may be temporarily, but abruptly, blocked by an obstacle and even lost when the terminal device moves or changes its posture. Accordingly, sudden service interruption may be incurred.
The conventional hard handover scheme is incompetent to avoid such service interruption, because the hard handover scheme requires a time-consuming process of transmitting control signaling across several radio access network components and the service interruption might have occurred before the transmitting process is completed.
Instead of using the hard handover scheme, the concept of terminal device-specific serving cluster (SvC) has been proposed to be used in high-frequency wireless communication systems for effecting faster switching of the terminal device between SANs. To be specific, a SvC refers to a group of SANs that are located in the vicinity of a terminal device and are ready to serve the terminal device. When a serving beam between an SAN in the SvC and the terminal device is blocked, communications carried by the beam can be quickly switched over to another beam between a different SAN in the SvC and the terminal device. As such, continuous service provision is ensured by close cooperation among the group of SANs in the SvC and the risk of service interruption can be eliminated.
One of the SANs in the SvC, which conducts communications with the terminal device most of the time, is referred to as a Principal SAN (P-SAN). That is, the P-SAN handles the majority of data to be sent to and to be received from the terminal device. All the other SANs in the SvC are referred to Assistant SANs (A-SANs), whose responsibility is to temporarily take over the communications with the terminal device when a direct signal path between the P-SAN and the terminal device is lost (e.g. because an obstacle is present in the path).
The P-SAN is also responsible to request the A-SANs to measure signal qualities on links between the A-SANs and the terminal device and report the measurements to the P-SAN. Based on the reported measurements, the P-SAN makes a decision as to which of the A-SANs shall serve the terminal device when the link between the P-SAN and terminal device is lost.
Even when there is no data transmission from or to an A-SAN, the A-SAN should track the terminal device by periodically transmitting known pilot signals on certain test beams, as directed by the P-SAN, in some scheduled radio resources, so that the A-SAN can be readily aware of which beam to use to transmit to or receive from the terminal device when needed.
Although the concept of terminal device-specific SvC is theoretically feasible to be used for avoiding sudden service interruption in high-frequency wireless communication systems as explained above, it is not robust enough to work well in real systems.
To be specific, as illustrated in FIG. 1, SAN1 may serve as a P-SAN for UE1, and SAN2 may serve as an A-SAN for UE1 and meanwhile serve as a P-SAN for UE2. Serving as the P-SAN for UE2, SAN2 may autonomously determine to transmit a test beam to UE2 during timeslot 1. On the other hand, SAN1 that serves as the P-SAN for UE1 may instruct SAN2, which serves as the A-SAN for UE1, to receive a test beam from UE1 during the same timeslot. Thus, SAN2 is in a dilemma as to whether to transmit or receive a test beam during timeslot 1, because there is a conflict between the test beam transmission directions determined by SAN2 itself and indicated by SAN1. Also, SAN 2 is faced with a dilemma of whether to communicate a test beam with UE1 or UE2, because there is a conflict between the terminal devices to be tracked determined by SAN2 itself and indicated by SAN1.